Understanding Camera Zoom Lenses: Parfocal and Varifocal Specifications

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Camera lenses come in various types, one of which is the zoom lens. But not all zoom lenses are created equal. To further refine our understanding, there is a distinct category of zoom lenses known as parfocal and varifocal lenses, each offering a unique set of features essential for professional photography and videography. In this article, we will delve into the characteristics of parfocal lenses and discuss the differences with varifocal lenses.

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What is a Parfocal Lens?

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A zoom lens that maintains focus when changing focal lengths is called a parfocal lens. Parfocal lenses are specifically designed to keep the focus consistent while the zoom level is adjusted. This feature is invaluable in professional photography and videography where maintaining focus during a zoom is critical.

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Real-World Application

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Professional filmmakers and photographers use parfocal lenses because they ensure a seamless zooming experience. When switching between different focal lengths, the image remains sharply in focus, making it easier to focus and frame the shot throughout the process. For instance, if Alexia is setting up a shot, she would first zoom in and focus, then zoom out and start filming. This technique ensures that the focus error is not visible and is practically seamless.

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Modern Connotation and Misunderstandings

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In the modern context, a zoom lens is any lens that allows for a variable focal length. This term generally refers to any lens capable of altering magnification, not necessarily a telescopic one. However, there is a common misconception that some consumers confuse the terms zoom and telephoto. While this is not technically accurate, saying zoom lens often means variable magnification to most photographers.

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There are terms like telephotolens and optical zoom that are often incorrectly used by marketers and tech journalists. For example, the term optical zoom is often used to describe the software-based digital zoom found in smartphones, which is a significant misuse of language. It is essential to correct these misunderstandings to maintain accurate technical terminology.

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Differences Between Parfocal and Varifocal Lenses

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A varifocal lens is a lens that changes its focal length without maintaining focus. These lenses are commonly used in consumer cameras because they are more cost-effective and compact. However, they often result in a slight loss of focus as the zoom level changes, which can be problematic in certain situations, especially when focusing manually for video.

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Use in Photography and Videography

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Most modern still camera zoom lenses are varifocal, meaning they do not maintain constant focus. These lenses might lose focus as you adjust the focal length, which can be an issue in videography. On the other hand, lenses used for video and those designed for cinematography are often engineered to be parfocal or to simulate parfocal behavior. True cinematography lenses are not only parfocal but also do not breathe—meaning they do not change the effective focal length as the focus changes.

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Historically, in the days of manual focus, a good zoom lens was a parfocal lens with a fixed aperture at all focal lengths. This allowed photographers to set the focus at the widest focal length and maintain it throughout the shot. However, advancements in autofocus systems and electronic sensors have made varifocal lenses more practical and cost-effective.

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Advanced Knowledge: Breathing and Focus Compensation

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A significant aspect of lens design is the concept of breathing, which refers to the change in effective focal length as the focus changes. Cinematography lenses, in particular, use a focus compensation group that counters the breathing effect, allowing for precise framing. This technology is crucial for shots that demand precise composition, such as the legendary dolly zoom technique from Alfred Hitchcock's Vertigo.

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The Dolly Zoom Effect

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The dolly zoom or vertigo effect is a skillful technique used by cinematographers to create a distinctive visual effect. It involves moving a camera towards or away from a subject while zooming out or in, respectively. This effect is made possible with parfocal lenses, as any movement must be precise to maintain the subject's focus and framing. The dolly zoom technique is often associated with the psychological tension and disorientation in Vertigo.

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To summarize, understanding the difference between parfocal and varifocal lenses is essential for both amateur and professional photographers and videographers. While parfocal lenses provide seamless zooming with consistent focus, varifocal lenses offer the benefits of variable magnification at a lower cost. As technology advances, the use of more precise and advanced lens designs continues to evolve, making it even more critical to distinguish between these two types of lenses.